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Original poster

Secondary maths teachers - does this homework make sense?

28 replies

PointyChristmasFairyWand · 04/12/2013 19:14

DD1 has just spent an hour working out the cube root of 426 to 2 decimal points. She's convinced that she has to do all the calculations by hand - no calculators allowed. She understands the method perfectly well - work out between which two whole numbers it falls, then work out between which two numbers to one decimal it falls, then down to the 2 decimal point numbers.

She has spent an hour on one sum and has 2 more to do. One of them isn't a cube, it's a fourth power.

I don't see the point of doing it all by hand. I really, really don't. Everything I have looked up suggests that 'doing it by hand' will still involve using a calculator for the individual sums. Am I being dim? Is there a possibility that my DD has misunderstood and that it's OK to use a calculator for the 7.52x7.52x7.52 etc.? I get that it's useful to practice multiplying decimals, but this?

OP posts:
friday16 · 04/12/2013 23:15

Successive approximation is a slow way to find roots by hand, too, which you would never use in reality.

The fastest way to do cube roots by hand is probably Newton Raphson.

Root 426, initial guess 7.

7, 7.56, 7.52, 7.52, we're done.

At each stage you have to work out x - (x3-426)/(3x2). I used a 1981 programmable calculator, but it's hardly much slower using an ordinary calculator. As others have said, doing it completely by hand is not quite the point...

Original poster
PointyChristmasFairyWand · 04/12/2013 19:27

Thanks, that sounds much more plausible. DD is going to check with her maths teacher just in case, but as a trial and improvement exercise it makes total sense. She feels better now, and has another day to finish the work so it will all be fine. Flowers

OP posts:
mnistooaddictive · 04/12/2013 19:23

It sounds like trial and improvement and yes she can use the calculator. I think that when they said dont use the calculayor they meant dont use the cube root button! This would always be a calculayor question on an exam.

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